P. Onorato

Filtering of spin currents based on a ballistic ring

Quantum interference effects in rings provide suitable means for controlling spin at mesoscopic scales. Here we apply such a control mechanism to the spin dependent transport in a ballistic quasi-one-dimensional ring patterned in two-dimensional electron gases (2DEGs). The study is essentially based on the natural spin–orbit (SO) interactions, one arising from the laterally confining electric field (β term) and the other due to the quantum well potential that confines electrons in the 2DEG (conventional Rashba SO interaction or α term). We focus on single-channel transport and solve analytically for the spin polarization of the current. As an important consequence of the presence of spin splitting, we find the occurrence of spin dependent current oscillations. We analyze the transport in the presence of one non-magnetic obstacle in the ring. We demonstrate that a spin polarized current can be induced when an unpolarized charge current is injected in the ring, by focusing on the central role that the presence of the obstacle plays.

Characterization of aluminium nitride nanostructures by XANES and FTIR spectroscopies with synchrotron radiation

We investigated different AlN nano-systems using spectroscopic methods. Experiments were performed at the Synchrotron Radiation Facility of the Laboratori Nazionali di Frascati using both XANES (x-ray absorption near edge spectroscopy) and FTIR (Fourier transform infrared spectroscopy) techniques in order to investigate materials with both interesting tribological and electronic properties. Comparisons have been performed between measurements by standard x-ray diffraction (XRD) and x-ray absorption (XRS) at the K-edge of Al, a spectroscopy method sensitive to the local order and correlated to the local and empty density of states of this wide band-gap semiconductor. Preliminary XAS simulations at the Al K edge are also presented. Correlations between XRD and XAS have been drawn, since x-ray absorption reveals structural information complementary to that addressed by x-ray diffraction. Moreover, a comparison has been performed by infrared (IR) absorption both in the mid- and in the far-IR ranges between different AlN forms: namely, powders, nanoparticles and nanotubes. Data clearly show changes connected with the electronic properties and the optical phonon modes of AlN nano-systems.

Micro-Raman study of the role of sterilization on carbon nanotubes for biomedical applications

AIM We investigate the effect of four different types of sterilization procedures on the structural properties and morphological features of single-wall carbon nanotube samples approachable by micro-Raman spectroscopy. Sterilization procedures (treatment in humid heat autoclave or ethylene oxide and irradiation with gamma-rays or UV light) are necessary in view of the use of carbon nanotube sterile samples in in vivo toxicity tests on laboratory rats. Micro-Raman spectroscopy allows us to estimate several details about the morphology of the single-wall carbon nanotube mixture (mainly the presence of disorder and diameter distribution) before and after the sterilization treatment. RESULTS The best of these treatments, in other words, the one that least affected the morphology and structural properties of carbon nanotubes, was found to be UV irradiation and has thus been selected for future in vivo tests on rats.

Crossover from the Luttinger-liquid to Coulomb-blockade regime in carbon nanotubes.

We develop a theoretical approach to the low-energy properties of one-dimensional electron systems aimed to encompass the mixed features of Luttinger-liquid and Coulomb-blockade behavior observed in the crossover between the two regimes. For this aim, we extend the Luttinger-liquid description by incorporating the effects of a discrete single-particle spectrum. The intermediate regime is characterized by a power-law behavior of the conductance, but with an exponent oscillating with the gate voltage, in agreement with recent experimental observations. Our construction also accounts naturally for the existence of a crossover in the zero-bias conductance, mediating between two temperature ranges where the power-law behavior is preserved but with a different exponent.

Rashba effect in two-dimensional mesoscopic systems with transverse magnetic field

We present semiclassical and quantum mechanical results for the effects of a strong magnetic field in quantum wires in the presence of Rashba spin-orbit coupling. Analytical and numerical results show how the perturbation acts in the presence of a transverse magnetic field in the ballistic regime and we assume a strong reduction of the backward scattering interaction which could have some consequences for the Tomonaga-Luttinger transport. We analyze the spin texture due to the action of spin-orbit coupling and magnetic field, often referring to the semiclassical solutions that magnify the singular spin polarization: results are obtained for free electrons in a two-dimensional electron gas and for electrons in a quantum wire. We propose the systems as possible devices for the spin filtering at various regimes.

Magnetic field effects in carbon nanotubes

We study the effects of a perpendicular magnetic field on the transport properties of carbon nanotubes. For values of the magnetic length smaller than the curvature radius, the system displays well-defined Landau levels and an integer quantum Hall effect. The localized Gaussian Landau states develop in the central region of the nanotube surface, where the component of the magnetic field is maximum. Conversely, chiral currents flow at the flanks of the nanotube, producing a quantization of the Hall conductivity by even multiples of 2e2/h. Remarkably, it differs from the quantization rule by odd multiples of 2e2/h recently found in planar graphene. Finally, the effects of the electron?electron interaction in the quantum Hall regime are considered. It is shown that the localization of chiral currents on opposite sides of the system reflects a strong suppression of back-scattering, enhancing the tunnelling density of states.

Single-wall nanotubes: Atomiclike behavior and microscopic approach

Recent experiments about the low temperature behaviour of a Single Wall Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero bias conductance and allowed us to investigate the energy levels of interacting electrons. Other experiments confirmed the theoretical prediction about the crucial role which the long range nature of the Coulomb interaction plays in the correlated electronic transport through a SWCNT with two intramolecular tunneling barriers. In order to investigate the effects on low dimensional electron systems due to the range of electron electron repulsion, we introduce a model for the interaction which interpolates well between short and long range regimes. Our results could be compared with experimental data obtained in SWCNTs and with those obtained for an ideal vertical Quantum Dot (QD). For a better understanding of some experimental results we also discuss how defects and doping can break some symmetries of the bandstructure of a SWCNT.

Quantum Hall effect in carbon nanotubes and curved graphene strips

We develop a long-wavelength approximation in order to describe the low-energy states of carbon nanotubes in a transverse magnetic field. We show that in the limit where the square of the magnetic length

Investigating the role of sliding friction in rolling motion: a teaching sequence based on experiments and simulations

l=\sqrt{\ensuremath{\hbar}c∕eB}

Integer spin Hall effect in ballistic quantum wires

is much larger than the C-C distance times the nanotube radius